Significant boost of the stability and PLQYof CsPbBr3 NCs by Cu-BTC MOF

Developing stable perovskite nanocrystals (NCs) with enhancing luminescent properties holds great importance for future potential applications in optoelectronics. Here, we engaged perovskite NCs in Cu2+ ion-based metal–organic framework (MOF) Cu-BTC (BTC = 1,3,5-benzene tricarboxylate) by physical mixing of MOF with CsPbBr3 NCs in toluene solution. MOF-protected perovskite NCs achieved high photoluminescence quantum yield 96.51% than pristine state CsPbBr3 NCs (51.66%). Along with the improvement in optical properties, the long-term stability of CsPbBr3 NCs in the solution phase also increases considerably upon loading in Cu-BTC MOF. Moreover, the changes in the luminescent intensity of the samples have been observed for 3 months in the solution. After 1 month, pristine CsPbBr3 NCs lose their emission intensity 68% from the initial, while the MOF-protected CsPbBr3 NCs show only a 10% reduction from the initial. These results indicate that the effective passivation of Cu-BTC MOF inhibits the aggregation of NCs, protecting them from the defective atmosphere. The excellent photoluminescence findings provide a new pathway for future optoelectronic applications.

The Cu-BTC MOF was prepared by constantly stirring aqueous copper (II) nitrate solution with ethanolic 1,3,5-benzene tricarboxylic acid (BTC) solution at RT. For the synthesis of CsPbBr 3 @Cu-BTC nanocomposite, MOF powder was added into perovskite containing toluene solvent with a constant stirring at RT. The whole synthesis process is speedy, easy, and performed in the open air at room temperature. The significantly high PLQY 96.51% was obtained in CsPbBr 3 NCs, upon treating with Cu-MOF, while 51.66% in untreated CsPbBr 3 NCs. The Cu-BTC MOF enhanced the PLQY and improved the chemical stability of CsPbBr 3 NCs for months.

Results and discussion
Absorption spectra (Fig. 2a) of CsPbBr 3 NCs show a band at 509 nm and CsPbBr 3 NCs@Cu-BTC exhibit an absorption onset at 506 nm. As shown in Fig. 2b, CsPbBr 3 NCs display an emission peak at 519 nm in the photoluminescence spectral studies, while CsPbBr 3 NCs@Cu-BTC shows a highly intense emission band at 512 nm. The blue shift was observed in CsPbBr 3 NCs in both the cases absorption and emission upon loading into MOF, which may be attributed to a decrease in the size of NCs after engaging with MOF. Zhang and their co-workers have observed the blue shift in the photoluminescence spectra of MAPbI 2 Br after the formation of MAPbI 2 Br@ HKUST-1 composite 28 . The tunability in optical properties is the results of quantum confinement 29 .
The PL spectrum shows full width at half maximum (FWHM) of about 18 nm for pristine CsPbBr 3 NCs, narrower than that of guest CsPbBr 3 NCs (23 nm). The change in FWHM suggests that the intercalating of CsPbBr 3 NCs into the rigid porous MOF framework possibly resulted in a slight shift in particle size distribution. That meant the particle size is narrower by the intercalating process 30 . The absolute PLQYs of samples were recorded at 350 nm excitation wavelength. PLQY of 51.66% was measured in CsPbBr 3 NCs, while 96.51% PLQY was achieved in CsPbBr 3 NCs@Cu-BTC, hugely higher than pristine CsPbBr 3 NCs. It should be due to the more effective surface passivation of CsPbBr 3 NCs by terminal oxygen of Cu-BTC MOF decreasing surface defect density. The improvement in PLQY as well as in emission lifetime may be due to the surface defects passivation of perovskite by Cu-BTC. In the previous reports researchers have treated the surface halide vacancies by R-COOtype passivating ligand, increasing PLQY and emission lifetime of halide perovskites 31 . Pan et al. have reduced the surface trap state of perovskite NCs with 2,2′-iminodibenzoic acid achieving high PLQY and improve emission lifetime of treated perovskite NCs 32 . 96.51% PLQY of CsPbBr 3 @Cu-BTC composite is the highest PLQY than other reports for perovskite@MOF composite system 32,33 .
The Fluorescent microscopy images in Fig. 2c and Fig. S1 of CsPbBr 3 NCs@Cu-BTC microrod show many bright green fluorescent NCs spread over the Cu-BTC rod, verifying the association of CsPbBr 3 NCs with Cu-BTC MOF. The photoluminescence decay of samples can be described by triexponential fitting kinetics, as shown in Fig. 2d. CsPbBr 3 NCs@Cu-BTC composite was displayed a short-lived emission lifetime (τ 1 ) of 4 ns, midwaylived emission lifetime (τ 2 ) of 22 ns, and long-lived PL lifetime (τ 3 ) of 113 ns. The average lifetime (τ avg ) of 103 ns was observed for CsPbBr 3 NCs@Cu-BTC, which is significantly higher than that of the average lifetime (τ avg ) 66 ns for pristine CsPbBr 3 NCs. The improvement in the emission lifetime of CsPbBr 3 NCs@Cu-BTC composite should be due to the suppression of the nonradiative recombination pathway of CsPbBr 3 NCs by Cu-BTC MOF. These results are strongly consistent with the high PLQY of the composite than pristine CsPbBr 3 NCs.
Furthermore, the emission behaviour of samples immersed in toluene solution was recorded for 90 days to appraise the stability of CsPbBr 3 NCs. As shown in Fig. 3a, the fluorescence intensity decreases 68% from the first day to 30 days for pristine CsPbBr 3 NCs, while this loss was 10% for CsPbBr 3 NCs@Cu-BTC (Fig. 3b). In this order, after 90 days, pristine CsPbBr 3 NCs lost 98% of initial PL intensity, and MOF-protected CsPbBr 3 NCs still www.nature.com/scientificreports/ preserved 56% of their initial intensity (Fig. 3c). These results clearly show that the emission intensity decreasing rate was speedier for pristine CsPbBr 3 NCs than CsPbBr 3 @Cu-BTC composite. The mechanism of PL intensity decline can be discussed for both samples. In pristine CsPbBr 3 NCs, intensity decreases speedily due to the fast aggregation rate in toluene solution and decomposition of CsPbBr 3 NCs to their precursors. Interestingly, the red shift in emission spectra of aging pristine CsPbBr 3 NCs is directly related to the formation of trap states or defects, which can reduce the emission performance of NCs. Notably, in the case of protected CsPbBr 3 NCs with the comparison of pristine CsPbBr 3 NCs, very few emission intensity changes are observed. Cu-BTC inhibits the rate of aggregation and degradation of NCs, passivating trap states, or defects. These results indicate that the Cu-BTC MOF-treated CsPbBr 3 NCs exhibit better long-term stability than pristine CsPbBr 3 NCs. Powder X-ray diffraction (PXRD) patterns of CsPbBr 3 NCs, Cu-BTC, and CsPbBr 3 NCs@Cu-BTC composite were recorded as shown in Fig 36 . The minor additional peaks were obtained which may arise due to CuO impurities as reported in previous research. The peak intensity of the diffraction pattern may be affected by the environmental moisture during analysis 37 . The presence of CsPbBr 3 in the MOF can be ensured by observing the XRD pattern of the CsPbBr 3 NCs@Cu-BTC composite. In this diffraction pattern, the peaks of both CsPbBr 3 and Cu-BTC MOF are observed. Most of the peaks of CsPbBr 3 are overlapped with Cu-BTC peaks, but the peaks at 2θ = 15.27° and 37.79° corresponding to the (100) and (211) planes of the cubic CsPbBr 3 appeared. From these results, it is confirmed that CsPbBr 3 NCs were incorporated and well dispersed in Cu-BTC MOF. The stability test of storage samples immersed in toluene was examined by XRD up to 60 days (Fig. S4). The XRD pattern of pristine CsPbBr 3 NCs after aging shows a new peak at 12.81° with increasing intensity from 15 to 60 days, which can occur due to partial formation of Cs 4 PbBr 6 as an impurity in solution which may affect the emission 38 .
The peak height of cubic CsPbBr 3 gradually reduces up to 60 days, increasing sharpness. These changes in XRD spectra indicate the aggregation and degradation of CsPbBr 3 NCs consistent with PL results 13 . After storage, the peak at 30.68° become split which indicates the transformation of cubic phase to orthorhombic phase lead to the phase distortion of CsPbBr 3 NCs. This is also a reason for losing its optical performance after storage 39 .
The aggregation of CsPbBr 3 NCs in toluene solution proved by PXRD analysis as well as TEM studies after 60 days as shown in Fig. S4. In PXRD results, the diffraction peaks become narrower after 60 days as compared The morphology of the as-prepared CsPbBr 3 NCs, Cu-BTC MOF, and engaged CsPbBr 3 NCs with Cu-BTC MOF was investigated by transmission electron microscopy (TEM) analysis. As shown in Fig. 4a,b, the asprepared CsPbBr 3 shows nanoplate shape, with an average size of 13.12 nm (Fig. 4c), and the selective area electron diffraction (SAED) pattern of CsPbBr 3 NCs in Fig. 4d represents (200) and (210) reflection planes of the cubic structure of CsPbBr 3 . Figure 4e-g exhibit microrods morphology of Cu-BTC MOF. Figure 4h shows a highly crystalline SAED pattern of MOF in which a crystal face (553) was observed that was consistent with its characteristic diffraction pattern.  Figure 4k clearly shows that the CsPbBr 3 NCs are strewn over the Cu-BTC microrods. Moreover, the SAED pattern for CsPbBr 3 @Cu-BTC exhibits (210) and (440) crystal facets, in which (210) represents the cubic CsPbBr 3 and (440) is characteristic for Cu-BTC, as shown in Fig. 4l. The average particle size of CsPbBr 3 is 12.16 nm in the CsPbBr 3 @Cu-BTC composite (Fig. 4p). Meanwhile, the size of CsPbBr 3 decreased slightly in composite, which may be due to the surface passivation of NCs by BTC.
Further, the morphology of Cu-BTC MOF and CsPbBr 3 @Cu-BTC composite was observed by scanning electron microscopy (SEM), as shown in Fig. 5. The SEM images in Fig. 5a-c display microrods shape of Cu-BTC at different magnifications. When CsPbBr 3 NCs are loaded into Cu-BTC MOF, the SEM images for CsPbBr 3 @ Cu-BTC composite, as shown in Fig. 5d,e, represent a similar morphological pattern as Cu-BTC MOF. Figure 5f-l show the elemental mapping of synthesized CsPbBr 3 @Cu-BTC composite. In these results, the existence of Cs, Pb, and Br elements with C, O, and Cu indicates the uniform distribution of CsPbBr 3 NCs in Cu-BTC MOF.
The composition of elements was identified by energy-dispersive x-ray (EDX) analysis for Cu-BTC and CsPbBr 3 in Cu-BTC MOF. The EDX spectra (Fig. S6) at an arbitrary point of microrods show C, O, and Cu signals, indicating the distribution of these elements in Cu-BTC MOF construction. The CsPbBr 3 @Cu-BTC composites at a random point of microrods, the additional Cs, Pb, and Br signals are also obtained with the C, O, and Cu in the atom ratio of 1:1:3, which show the presence of CsPbBr 3 NCs (Fig. S7). These results illustrate the distribution of CsPbBr 3 NCs throughout the Cu-BTC microrods.
The porous behaviour of samples was investigated by Brunauer Emmett-Teller (BET) gas-sorption measurements. The N 2 adsorption-desorption isotherms of Cu-BTC and CsPbBr 3 @Cu-BTC composite as shown in Fig. S8 are revealing micropore nature. The pore size distribution was calculated by using the Barrett-Joyner-Halenda www.nature.com/scientificreports/ (BJH) method. The curves of pore size distribution as shown in Fig. S8b and d exhibit microporous range. The surface area and pore volume for Cu-BTC were calculated 821.30 m 2 /g and 0.461 cm 3 /g and for CsPbBr 3 @ Cu-BTC composite 565.73 m 2 /g and 0.316 cm 3 /g, respectively. The decreasing surface area and pore volume in CsPbBr 3 @Cu-BTC composite clearly indicate the incorporation of CsPbBr 3 NCs with Cu-BTC 41 . In this report, the obtained pore volume and surface area of Cu-BTC are almost similar to Ahmed et al. 42 and Peedikakkal et al. 43 reports.
FTIR studies were carried out as shown in Fig. S9. The surface binding ligands (oleic acid and oleylamine) in CsPbBr 3 NCs were disclosed by their characteristic vibrational signals. The vibrational peak at 2922 cm −1 reveals the stretching vibration of the C-H bond in -CH 3 and the peak of 2856 cm −1 is due to the C-H bond in -CH 2 of the aliphatic hydrocarbon chain. The peak of 1629 cm −1 is due to the N-H bending vibration for the NH 2 group of oleylamine. Moreover, the peak at 1563 cm -1 is due to the stretching vibration in -COO of oleic acid. In this contrast, for Cu-BTC, the peak at 741 cm -1 attributes to the Cu-O bond, which confirms the metal-ligand coordination. A band at around 1619 cm -1 indicates the symmetric stretching of C=O groups in BTC, and the peak at 1380 cm -1 represents C=C of benzene. These all characteristic vibrational signals confirm the formation www.nature.com/scientificreports/ of Cu-BTC 44 . The vibrational spectra for CsPbBr 3 NCs@Cu-BTC composite, the surface ligand peaks of CsPbBr 3 NCs cannot be differentiated due to the strong overlapping of Cu-BTC peaks. XPS analytical technique was performed to identify the chemical compositional elements of CsPbBr 3 @Cu-BTC compared with Cu-BTC MOF and CsPbBr 3 as shown in Fig. S10. For Cu-BTC, the spectrum only shows the binding energy peaks of C1s, O1s, and Cu2p, which are characteristic peaks for Cu-BTC MOF. The XPS spectrum of CsPbBr3@Cu-BTC exhibits additional peaks of Cs3d, Pb4f., and Br3d with C1s, O1s, and Cu2p binding energy peaks, which suggest the formation of CsPbBr 3 @Cu-BTC composite. It is worth noting that the negative binding energy shift of Pb4f. was observed for CsPbBr 3 @Cu-BTC compared to pristine CsPbBr 3 (Fig. S11), which may be due to the strong interaction of Pb 2+ with BTC ligand 45 .
The optical performance of CsPbBr 3 NCs has been improved by preventing its degradation and aggregation using a variety of surface ligands 16,46 . In the CsPbBr 3 @Cu-BTC composite, the oxygen atoms of the BTC linker present in the MOF may bind the CsPbBr 3 NCs, which results from the emergence of highly stable CsPbBr 3 NCs with low surface defects. The strong interaction of CsPbBr 3 NCs with Cu-BTC MOF is supported by the TEM images. However, the UV light field microscopic images show the high fluorescent surface decorated CsPbBr 3 @ Cu-BTC composite.

Conclusion
In summary, we showed the experimental realization of as-synthesized bright green luminescent CsPbBr 3 @Cu-BTC composite at ambient conditions. Cu-BTC MOF was used as a host to protect the degradation of CsPbBr 3 NCs in the solution system. Moreover, the obtained CsPbBr 3 @Cu-BTC exhibited PLQY of 96.51% with excellent luminescent property relative to that of 51.66% of pristine CsPbBr 3 NCs. Therefore, our approach significantly improves the long-term stability and optical properties of perovskite NCs by forming composite materials.

Experimental details
Chemicals. Cesium bromide, lead (II) bromide,copper (II) nitrate trihydrate, oleic acid, and oleylamine were purchased from Sigma Aldrich. Toluene, ethanol, and N,N-dimethylformamide (DMF) were obtained from Thomas Baker. 1,3,5-Benzene tricarboxylic acid (BTC) was purchased from TCI. All the chemicals were used as received without further purification.  Instrumentation. Shimadzu UV-Vis 2450 Spectrophotometer was used for recording the UV-Vis absorption spectra in the range of 460-600 nm. Photoluminescence spectra were performed by using Horiba Scientific Fluoromax-4C Spectrophotometer. A quartz cuvette of 10 mm path length and volume of 3 ml was used forcollecting the spectra.Quantum yield as an absolute quantum yield was measured directly by using Edinburgh instruments FLS 980. Powder XRD was carried out in a Bruker-D8 using Cu-Ka radiation with an accelerating voltage of 40 kV from 5° to 50° with a rate of 1°/min. The thin film samples were prepared on silica glass.Transmission electron microscopy (TEM) studies were carried out using TECHNAI G2 20 S-TWIN. These were performed by taking a drop of highly diluted sample on a carbon-coated copper grid and drying it at 80 °C under vacuum overnight before analysis. Field emission scanning electron microscopy (FESEM) images were recorded for all the samples by Carl Zeiss Ultra Plus. The thin film samples were prepared on silica glass and drying it at 80 °C under vacuum overnight. N 2 adsorption-desorption isotherms were measured using a Micromeritics ASAP 2020 adsorption analyzer at 77 K. Fourier transform infrared spectroscopy (FTIR)spectra of all the samples were recorded by using Thermo Scientific Nicolet 6700. X-ray photoelectron spectroscopy (XPS) analysis of the thin film of samples prepared on silica glass was studied using PHI 5000 Versa Probe III.